Recent advances in the delivery of peptide-based therapeutics have raised new interest in the use of peptide-based vaccines. Synthetic peptides offer several advantages over other forms of vaccines based on inactivated or attenuated microorganisms, including easy production, high safety and low, or even no, detrimental immune responses usually associated with inactivated or attenuated microorganisms. Peptide immunogens have been widely used to induce antibodies with known epitope specificities. Thus, the epitope specificities of the resultant antibodies are determined by the immunologically dominant residues and sometimes also the flank sequences. However, often these antibodies against the immunodominant sequences lack therapeutic importance because those immunodominant epitopes are subject to high degree of mutation in dealing with highly variable viruses, such as HIV, hepatitis C virus (HCV).
In the case of human immunodeficiency virus type 1(HIV-1) infection, the major antibody responses are directed to variable gp120 regions that the virus uses as “decoy” when it mutates and still retains replicating capability. The conserved elements are less immunodominant and do not induce strong antibody response. However, evidence has shown that antibody responses to these conserved elements are critical for containing the virus. Previous studies characterized the third variable (V3) loop of the envelope gp120 as the principal neutralizing determinant (PND) for laboratory T-cell-line-adapted (TCLA) strains of HIV-1. However its amino acid sequence was highly variable so that the mutant resistant to neutralizing antibodies readily arises, thus limiting the potential of targeting this region as a vaccine strategy. Among four residues at the tip of the PND, only the three amino acid residues glycine, proline, and glycine (GPG) in the crown are highly conserved while the fourth residue is variable; A comparison of more than 2000 HIV-1 envelope sequences has shown that more than 95% of their V3 sequences contain the GPG motif while the fourth residue is variable with R in 34%, Q in 54% and K in 4% of the sequences. Functional studies have demonstrated that the PND is a major determinant of viral tropism and a single amino acid substitution within the motif can abrogate viral infectivity in SupT1 and CEM cells, suggesting the conservation of this specific sequence being critical to viral functionality. Serological studies of naturally infected patients have so far only identified antibody specific to all four residues and various reported immunization studies have not shown the ability to induce antibodies specific for the GPG sequence in the PND, regardless of the immunogens used in those studies, such as virus like particle (VLP), recombinant gp120 and peptides, or different immunization strategy taken, such as cocktail peptide, a chimeric virus library Human rhinovirus (HRV), and sequential immunization with V3 peptide. There is no report of antibodies that are specific to GPG itself. Thus, the fact that the critically conversed sequences, such as GPG in the case of HIV, may hold the key to fight against the virus and yet it is so far proved to be difficult to design immunogens for raising the antibodies targeting the GPG motif per se with less concern about mutation, demonstrated the need for novel methods of making vaccines to produce antibodies against the epitope sequences that are not very immunodominant but therapeutically significant, such as the GPG motif.